Patrick Bamford struck a hat-trick on the night.
Smith said, "I thought it was fairly even in the first half in terms of chances but you have to make the most of your moments.
"I was disappointed after the first goal - I thought we lost the game after the first goal, we got disillusioned and they were very good.
"But after five games we have 12 points. Credit to Leeds, their work-rate was very good.
"They got their first goal and that changed the game. We looked disappointed and surprised.
"We didn't handle it very well and it's something to learn from. If someone offered 12 points from five games we would have taken it. We knew we could make history today with five wins [at the start of a season] and we're disappointed we haven't done that."